The present invention relates to chemicals that inhibit thermal runaway in batteries, including flame-retarding electrolytes, as well as a method for preparing and using them.
Batteries (often termed cells or electrochemical cells) are devices in which redox reactions occur at each of two electrodes separated by an ionically-conductive medium called an electrolyte, which can be either solid or liquid. During discharge, reduction occurs at the positive electrode (also called the cathode) and oxidation occurs at the negative electrode (known as the anode). Discharge is a spontaneous cell reaction where energy is released and can be used to supply power. During charging, voltage is supplied to the cell to store chemical energy at the electrodes. Primary batteries are those that cannot be recharged, i.e., used once and discarded. Secondary (commonly known as rechargeable) batteries involve chemically reversible reactions and can be recharged many times.
Lithium has the lowest redox potential of all metals, is very light-weight, and is non-toxic in oxidized form. For these reasons, lithium has been widely studied and utilized as an anodic material. Lithium rechargeable batteries that use liquid non-aqueous electrolytes are now under intensive research because they exhibit enhanced properties, such as high ionic conductivity, high output voltage and increased capacity.
Currently employed liquid non-aqueous electrolytes consist of two components: an organic solvent and a lithium ion source. A commonly used organic solvent is ethylene carbonate (xe2x80x9cECxe2x80x9d) because of its low cost, good electrochemical stability, and high dielectric constant, which facilitates the dissolution of the lithium ion source and contributes to high ionic conductivities. Other carbonates, such as dimethyl carbonate (xe2x80x9cDMCxe2x80x9d) and propylene carbonate (xe2x80x9cPCxe2x80x9d) are often used in conjunction with EC to reduce viscosity, as well as to increase the wettability of the electrolytic solution with battery components, e.g., separator and electrodes. The lithium ion source is generally a salt with LiPF6 being a preferred salt because of its ease of dissolution (ca. 1 M) in carbonates and low cost compared to that of fluorinated salts, such as CF3SO3Li and (CF3SO2)2NLi. Liquid electrolytes comprised of a EC-DMC mixture (1:1) and LiPF6 exhibit ionic conductivities greater than 10xe2x88x923 S/cm at room temperature, a prerequisite for reliable operations of lithium-ion batteries.
Small lithium rechargeable batteries (also called lithium-ion batteries), which possess high energy density compared to other secondary batteries, are commercially available (with a capacity of 1300 to 1900 mAh) to power portable electronic devices such as cellular phones, camcorders, computers and cameras. Full size lithium-ion batteries are now under consideration for use in electric vehicles (EVs) to provide a longer driving range, higher acceleration, longer lifetime and a reduction in environmental pollution. Mass scale production of these batteries will be hampered until the safety-related issues, including controlling thermal runaway, are addressed.
For instance, under abusive conditions (e.g., shorting, crushing, or excessive over-charging) and occasionally under normal conditions (e.g., over-discharge, resistive and/or forced over-discharge), lithium-ion batteries that do not include safety features undergo thermal runaway. Thermal runaway is the condition where the rate of heat generation within a battery exceeds the battery""s (and its operating environment""s) capacity to dissipate the heat. This condition can cause accelerated dryout and increased charging current acceptance, which will eventually result in the battery igniting and/or exploding. In consumer oriented lithium-ion batteries, manufacturers employ external safety devices to minimize these potential hazards.
These devices include smart charge control (to avoid over-charge), a poly-thermal switch (to respond to a temperature rise in the battery), current path interrupter (to respond to a rise in internal pressure), and an aluminum rupture disk (as an over-pressure disconnect). These safety devices are expensive and are not cost effective for use in EVs, in which large volumes of liquid electrolytes are required.
Lithium rechargeable batteries also use another type of a non-aqueous electrolyte, a polymer-gel (often termed gel) electrolyte, in which comparatively less liquid electrolyte is used in a cross-linked polymer matrix. Because polymer-gel electrolyte requires less liquid, it reduces the magnitude of explosion or fire, but thermal runaway of the liquid electrolyte remains a critical concern. On the other hand, lithium rechargeable batteries containing a solid polymer electrolyte are comparatively safer, lighter in weight, more compact in size, and offer a more flexible design. Unfortunately, lithium polymer rechargeable batteries are not suitable for commercial production because of low ionic conductivities at room temperature. See e.g., Linden, D., Ed., Handbook of Batteries and Fuel Cells, McGraw-Hill, NY, 1995. Accordingly, there remains a need to provide safer electrolytes in batteries while maintaining high ionic conductivities. Such batteries require an electrolyte that inhibits, and preferably prohibits, thermal runaway and that is compatible with existing electrode and battery fabrication technology.
To improve the safety of these batteries, a number of approaches have been proposed. For example, Japanese Patent No. 7,192,762 discloses adding a halogenated formate ester to a nonaqueous electrolyte to decrease the flammability of the electrolyte. Further, European Application No. EP 0938151 discloses the use of a variety of fluorinated compounds in carbonate solvents to reduce the flammability of lithium battery electrolytes. U.S. Pat. No. 5,803,600 discloses the addition of certain compounds (e.g., phospholanes, cyclophosphazenes, silanes) to a variety of carbonates that generate CO2 upon decomposition, thereby reducing the flammability of battery electrolytes. Unfortunately, the foregoing approaches require very large quantities (e.g., 20-80% by weight) of the flame-retarding additives to ensure proper operation. U.S. Pat. No. 6,077,624 discloses reducing thermal runaway in vinylidene fluoride copolymer-based electrochemical cells by decreasing the number of reactive sites of the copolymer, which is effectuated through cross-linking the copolymer in order to selectively dehydroflorinate the vinylidine flouride units in the copolymer.
However, one problem is that none of the foregoing references teach commercially lithium-ion batteries of sufficient size and safety to power a variety of mechanical or electrical devices such as an EV. None of the references teach inhibiting thermal runaway by using a chemical interference mechanism via free radical scavenging and/or fire damping through flame resistant coatings. A further problem is that prior thermal runaway inhibitors were required to be present at relatively high percentages (e.g., 20-80% by weight) in the electrolyte. The present invention is provided to solve these and other problems.
The present invention provides for a battery comprising an anode, a cathode, and a flame-retarding electrolyte having a conductivity greater than about 10xe2x88x923 S/cm at ambient temperature and which includes a compound that chemically interferes with flame propagation. In one embodiment, the compound that chemically interferes with flame propagation is a free radical scavenger or a fire damping compound, or a combination thereof. The present invention also provides that the compound that comprises the thermal runaway inhibitor has the general chemical structure: 
where X is either an oxygen atom or a sulfur atom, and R1 and R2 are independently selected from the group consisting of (a) C, to C12 alkyl or haloalkyl moieties that may be terminally substituted, (b) C5 to C7 aryl moieties that are possibly substituted, and (c) trialkylsilyl moieties in which alkyl groups contain 1 to 6 carbon atoms.
The present invention further provides that Z is a moiety selected from the group consisting of aryl, aralkylene, dialkylamino, diarylamino, alkylarylamino, trialkyleneamino, cyclic amino, cyclic amido, cyclic imido, and oxy derivatives thereof, as well as tetraalkyleneoxysilane and alkylalkyleneoxysilane. Further, m is an integer 1 to 4.
In another embodiment, the compound that comprises the thermal runaway inhibitor has the general chemical structure: 
In this embodiment, X is an oxygen atom or a sulfur atom, and R is selected from the group consisting of moieties of (a) C1 to C12 alkyl or haloalkyl moieties that may be terminally substituted, (b) C5 to C7 aryl moieties that are possibly substituted, and (c) trialkylsilyl moieties in which alkyl groups contain 1 to 6 carbon atoms. The Z1 and Z2 moieties are each independently selected from the group consisting of aryl, aralkylene, dialkylamino, diarylamino, alkylarylamino, trialkyleneamino, cyclic amino, cyclic amido, cyclic imido, and oxy derivatives thereof.
The present invention also provides a method for synthesizing the compounds that comprise the thermal runaway inhibitors in a flame-retarding electrolyte, as well as exemplary apparatuses for employing the invention.